I. Field of the Invention
This invention relates to a process for the preparation of liquid hydrocarbons from synthesis gas. In particular, it relates to a process wherein C.sub.10 + distillate fuels, and other valuable products, are prepared by reaction of carbon monoxide and hydrogen, over certain types of cobalt catalysts.
II. The Prior Art
Methane is often available in large quantities from process streams either as an undesirable by-product in admixture with other gases, or as an off gas component of a process unit, or units. More importantly, however, methane is the principle component of natural gas, and it is produced in considerable quantities in oil and gas fields. The existence of large methane, natural gas reserves coupled with the need to produce premium grade transportation fuels, particularly middle distillate fuels, creates a large incentive for the development of a new gas-to-liquids process. The technology to convert coal or natural gas to synthesis gas is well established, and the conversion of the synthesis gas to hydrocarbons can be carried out via Fischer-Tropsch synthesis.
Fischer-Tropsch synthesis for the production of hydrocarbons from carbon monoxide and hydrogen is now well known in the technical and patent literature. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co:18 ThO.sub.2 :100 kieselguhr, parts by weight, and over the next few years by catalysts constituted to 100 Co:18 ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200 kieselguhr, respectively. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates. Most commercial experience has been based on cobalt and iron catalysts. The cobalt catalysts, however, are of generally low activity necessitating a multiple staged process, as well as low synthesis gas throughput. The iron catalysts, on the other hand, are not really suitable for natural gas conversion due to the high degree of water gas shift activity possessed by iron catalysts. Thus, more of the synthesis gas is converted to carbon dioxide in accordance with the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x CO.sub.2 ; with too little of the synthesis gas being converted to hydrocarbons and water as in the more desirable reaction, represented by the equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O.
There exists a need in the art for a process useful for the conversion of synthesis gas at high conversion levels, and at high yields to premium grade transportation fuels, especially C.sub.10 + distillate fuels; particularly without the production of excessive amounts of carbon dioxide.
III. Objects
It is, accordingly, a primary objective of the present invention to supply this need.
A particular object is to provide a novel process useful for the conversion of synthesis gas, i.e., carbon monoxide and hydrogen to high quality distillate fuels characterized generally as admixtures of C.sub.10 + linear paraffins and olefins.
IV. The Invention
These objects and others are achieved in accordance with the present invention embodying a process wherein an admixture of carbon monoxide and hydrogen is contacted over a cobalt catalyst, especially a thoria promoted cobalt catalyst, formed by dispersing the cobalt, or thoria and cobalt, upon a titania or titania-containing support wherein the titania is one having a rutile:anatase weight ratio of at least about 2:3, and preferably at least about 3:2 to produce, at reaction conditions, a distillate fuel constituted principally of an admixture of linear paraffin and olefins, particularly a C.sub.10 + distillate which can be further refined and upgraded to high quality fuels, and other products such as mogas, diesel fuel, jet fuel, lubes and specialty solvents, especially premium middle distillate fuels of carbon numbers ranging from about C.sub.10 to about C.sub.20. The rutile:anatase ratio is determined by ASTM D 3720-78: Standard Test Method for Ratio of Anatase to Rutile In Titanium Dioxide Pigments By Use of X-Ray Diffraction.
The cobalt-titania catalyst, or thoria promoted cobalt-titania catalyst used in this process, is one wherein cobalt, or cobalt and thoria, is composited, or dispersed upon titania, TiO.sub.2, or a titania-containing carrier, or support, wherein the support contains a rutile:anatase ratio of at least about 2:3, and preferably at least about 3:2. In its most preferred form the titania, or titania component of the carrier, or support, will contain a maximum of rutile TiO.sub.2, as opposed to the anatase or other form of titania; generally a rutile:anatase ratio of from about 3:2 to about 100:1, or greater, and more preferably from about 4:1 to about 100:1, and greater. The cobalt, or cobalt and thoria, is dispersed on the support in catalytically effective amounts. Suitably, in terms of absolute concentration, the cobalt is dispersed on the support in amounts ranging from about 2 percent to about 25 percent, preferably from about 5 percent to about 15 percent, based on the total weight of the catalyst composition (dry basis). The thoria is dispersed on the support in amounts ranging from about 0.1 percent to about 10 percent, preferably from about 0.5 percent to about 5 percent, based on the total weight of the catalyst composition (dry basis). Suitably, the thoria promoted cobalt catalyst contains Co and ThO.sub.2 in ratio of Co:ThO.sub.2 ranging from about 20:1 to about 1:1, preferably from about 15:1 to about 2:1, based on the weight of the total amount of Co and ThO.sub.2 contained on the catalyst. These catalyst compositions, it has been found, produce at reaction conditions a product which is predominately C.sub.10 + linear paraffins and olefins, with very little oxygenates. These catalysts provide high selectivity, high activity and good activity maintenance in the conversion of carbon monoxide and hydrogen to distillate fuels.
In conducting the reactions the total pressure upon the reaction mixture is generally maintained above about 80 pounds per square inch gauge (psig), and preferably above about 140 psig, and it is generally desirable to employ carbon monoxide, and hydrogen, in molar ratio of H.sub.2 :CO above about 0.5:1 and preferably equal to or above 2:1 to increase the concentration of C.sub.10 + hydrocarbons in the product. Suitably, the H.sub.2 :CO molar ratio ranges from about 0.5:1 to about 4:1, and preferably the carbon monoxide and hydrogen are employed in molar ratio H.sub.2 :CO ranging from about 2:1 to about 3:1. In general, the reaction is carried out at gas hourly space velocities ranging from about 100 V/Hr/V to about 5000 V/Hr/V, preferably from about 300 V/Hr/V to about 1500 V/Hr/V, and at temperatures ranging from about 160.degree. C. to about 290.degree. C., preferably from about 190.degree. C. to about 260.degree. C. Pressures preferably range from about 80 psig to about 600 psig, more preferably from about 140 psig to about 400 psig. The product generally and preferably contains 60 percent, or greater, and more preferably 75 percent, or greater, C.sub.10 + liquid hydrocarbons which boil above 160.degree. C. (320.degree. F.).
Cobalt-titania, and especially thoria promoted cobalt-titania catalysts exhibit high activity and selectivity in the conversion of carbon monoxide and hydrogen to C.sub.10 + distillate fuels. The catalysts employed in the practice of this invention are prepared by techniques known in the art for the preparation of these and other catalysts. The catalyst can, e.g., be prepared by gellation, or cogellation techniques. Suitably, however, cobalt can be composited alone, or with the thoria, upon a previously pilled, pelleted, beaded, extruded, or sieved titania or titania-containing support material by the impregnation method. In preparing catalysts, the metal, or metals, is deposited from solution on the support to provide the desired absolute amount of the metal, or metals. Suitably, the cobalt is composited with the support by contacting the support with a solution of a cobalt-containing compound, or salt, e.g., a nitrate, carbonate or the like. The thoria, where thoria is to be added, can then be composited with the support in similar manner, or the thoria can first be impregnated upon the support, followed by impregnation of the cobalt. Optionally, the thoria and cobalt can be coimpregnated upon the support. The cobalt compounds used in the impregnation can be any organometallic or inorganic compound which decomposes to give cobalt oxide upon calcination, such as cobalt nitrate, acetate, acetylacetonate, naphthenate, carbonyl, or the like. Cobalt nitrate is especially preferred while cobalt halide and sulfate salts should generally be avoided. The salts may be dissolved in a suitable solvent, e.g., water, or hydrocarbon solvent such as acetone, pentane or the like. The amount of impregnation solution used should be sufficient to completely immerse the carrier, usually within the range from about 1 to 20 times the carrier by volume, depending on the concentration of the cobalt-containing compound in the impregnation solution. The impregnation treatment can be carried out under a wide range of conditions including ambient or elevated temperatures. Metal components other than thorium may also be added as promoters. Exemplary of such promoters are nickel, platinum, palladium, rhodium and lanthanium. In general, however, the addition of these metals have not been found to provide any significant benefit. In fact, surprisingly, the addition of copper and iron appear to have had a somewhat adverse effect upon the reaction. For this reason, the preferred catalyst is one which consists essentially of cobalt, or cobalt and thoria, dispersed upon the titania, or titania-containing support; or, in other words, catalysts which do not contain a significant amount of a metal, or metals, other than cobalt, or metals other than cobalt and thorium, dispersed upon the titania or titania-containing support.
Titania is used as a support, or in combination with other materials for forming a support. The titania used for the support, however, is necessarily one which contains a rutile:anatase ratio of at least about 2:3 , and preferably at least about 3:2, as determined by x-ray diffraction. Preferably, the titania is one containing a rutile:anatase ratio ranging from about 3:2 to about 100:1, and greater, preferably from about 4:1 to about 100:1, and greater. The surface area of such forms of titania are less than about 50 m.sup.2 /g. This concentration of rutile provides generally optimum activity, and C.sub.10 + hydrocarbon selectivity without significant gas and CO.sub.2 make.
The catalyst, after impregnation, is dried by heating at a temperature above about 0.degree. C., preferably between 0.degree. C. and 125.degree. C., in the presence of nitrogen or oxygen, or both, in an air stream or under vacuum. To obtain high activity, it is necessary to activate the cobalt-titania, or thoria promoted cobalt-titania catalyst prior to use. Preferably, the catalyst is contacted with oxygen, air, or other oxygen-containing gas at temperature sufficient to oxidize the cobalt and convert the cobalt to Co.sub.3 O.sub.4. Temperatures ranging above about 150.degree. C., and preferably above about 200.degree. C. are satisfactory to convert the cobalt to the oxide, but temperatures above about 500.degree. C. are to be avoided unless necessary for regeneration of a severely deactivated catalyst. Suitably, the oxidation of the cobalt is achieved at temperatures ranging from about 150.degree. C. to about 300.degree. C. The metal, or metals, contained on the catalyst are then reduced. Reduction is performed by contact of the catalyst, whether or not previously oxidized, with a reducing gas, suitably with hydrogen or a hydrogen-containing gas stream at temperatures above about 200.degree. C.; preferably above about 250.degree. C. Suitably, the catalyst is reduced at temperatures ranging from about 200.degree. C. to about 575.degree. C. for periods ranging from about 0.5 to about 24 hours at pressures ranging from ambient to about 40 atmospheres. A gas containing hydrogen and inert components in admixture is satisfactory for use in carrying out the reduction.
The cobalt, and thoria promoted cobalt-titania catalysts of this invention can be regenerated, and reactivated to restore their initial activity and selectivity after use by stripping the catalyst with a hydrocarbon solvent, or with a gas. Preferably the catalyst is stripped with a gas, most preferably with hydrogen, or a gas which is inert or non-reactive at stripping conditions such as nitrogen, carbon dioxide, or methane. The stripping removes the hydrocarbons which are liquid at reaction conditions. Gas stripping can be performed at substantially the same temperatures and pressures at which the reaction is carried out. Pressures can be lower however, as low as atmospheric. Temperatures can thus range from about 160.degree. C. to about 290.degree. C., preferably from about 190.degree. C. to about 260.degree. C., and pressures from about atmospheric to about 600 psig, preferably from about 140 psig to about 400 psig.
If it is necessary to remove coke from the catalyst, the catalyst can be contacted with a dilute oxygen-containing gas and the coke burned from the catalyst at controlled temperature below the sintering temperature of the catalyst. The temperature of the burn is controlled by controlling the oxygen concentration and inlet gas temperature, this taking into consideration the amount of coke to be removed and the time desired to complete the burn. Generally, the catalyst is treated with a gas having an oxygen partial pressure of at least about 0.1 psi, and preferably in the range of from about 0.3 psi to about 2.0 psi to provide a temperature ranging from about 300.degree. C. to about 550.degree. C., at static or dynamic conditions, preferably the latter, for a time sufficient to remove the coke deposits. Coke burn-off can be accomplished by first introducing only enough oxygen to initiate the burn while maintaining a temperature on the low side of this range, and gradually increasing the temperature as the flame front is advanced by additional oxygen injection until the temperature has reached optimum. Most of the coke can be readily removed in this way. The catalyst is then reactivated, reduced, and made ready for use by treatment with hydrogen or hydrogen-containing gas as with a fresh catalyst.